The present invention relates to an apparatus and method for detecting an in-cylinder pressure of an internal-combustion engine, and in particular to an apparatus and method for detecting a more accurate in-cylinder pressure by removing a drift from the in-cylinder pressure.
Conventionally, an in-pressure sensor is disposed in a cylinder of an engine to detect a pressure within the cylinder (referred to as an “in-cylinder pressure”). The in-cylinder pressure detected by such a sensor is used for controlling the engine.
An in-cylinder pressure sensor using a piezo-electric element is known. The piezo-electric sensor detects a change rate of the in-cylinder pressure. As shown in FIG. 25, the change rate detected by the in-cylinder pressure sensor 200 is typically integrated by an integration circuit 201. An output of the integration circuit 201 is used as an in-cylinder pressure.
In general, when a piezo-electric element is used, there is hysteresis between a change of the in-cylinder pressure and the output of the in-cylinder pressure sensor. The output of the in-cylinder pressure sensor increases with an increase in the temperature of the piezo-electric element. When such an in-cylinder pressure sensor is mounted on an engine, variations occur in the output of the in-cylinder sensor due to the heat from the engine. As a result, “deviation” or a drift as shown in FIG. 26 may appear in the waveform of the in-cylinder pressure obtained from the integration circuit.
If a drift occurs, it is difficult to detect a correct in-cylinder pressure. The output of the in-cylinder pressure sensor is typically converted from analog to digital (A/D conversion) for subsequent computer processes. If a drift component is contained in the output of the in-cylinder pressure sensor, correlation may be lost between an analog value that is an output of the in-cylinder pressure sensor and a digital value that is an output after the A/D conversion.
Japanese Patent Application Publication H07-280686 discloses a technique for correcting a drift by resetting the integration circuit. Referring to FIG. 27, a switching element 212 is closed at a predetermined timing in each combustion cycle of an engine. When the switching element is in a closed state, no voltage potential difference exists across a capacitor 213 and hence the output of an operational amplifier 214 is reset to a reference value. A drift is removed in response to the reset operation.
FIG. 28 shows a waveform of the in-cylinder pressure obtained from the integration circuit when the above reset operation is performed. The reset operation is carried out at time t1, t2, t3, t4 and t5. It is seen that a waveform 115, which is caused by the reset operation, is superimposed on the waveform of the in-cylinder pressure. In response to the reset operation, frequency discontinuities appear in the in-cylinder pressure waveform. Due to such frequency discontinuities, an undesired frequency component is introduced into subsequent computer processes, which reduces the accuracy in controlling the engine. Even if such reset operation is performed, a drift may increase during one combustion cycle (that is, between the reset operation at one time and the reset operation at another time).
Thus, there is a need for a technique for removing a drift to detect an in-cylinder pressure without performing such reset operation.